Process for the continuous electrolytic regeneration of spent iron containing sulfate pickling solution



United States Patent PROCESS FGR THE (IONTINUQUS ELECTRO- LYTlC REGENERATZON OF SEENT IRGN CQN- TAINING SULFATE PICKLLIG SOLUTION Rolf Sommer, Aswan, Egypt, Heinrich Miiller, Lndwigshafen (Rhine), Germany, and Peter Czedik-Eysenberg, Vienna, Friedrich Ganglherger, Stockerau, and Gerhard Jangg, Vienna, Austria, assignors to Wiener Schwachstromwerhe Gesellschaft m.h.H., Vienna, Austria No Drawing. Filed Mar. 17, 1964, Ser. No. 368,432 Claims priority, application Austria, June 11, 1958,

A 4,079/58 8 (Iiaims. (Cl. 204-104) This is a continuation-in-part application of our copending application Serial No. 812,102, filed on May 11,

1959, now abandoned.

The invention relates to a process for the continuous electrolytic regeneration of spent iron containing sulfate pickling solution.

Amalgam-metallurgical processes have found increasing interest in recent years. It has been found that those amalgams which can be formed and decomposed in a simple manner can be produced and processed on an industrial scale. The amalgams of the metals which are soluble in mercury at least to some extent are suitable for such operations.

On the other hand, the metals of the iron group form amalgams which consist essentially of suspensions of the metal concerned or of mercury compounds thereof which have only a low solubility in mercury so that they are hardly responsive to anodic decomposition. Nor can these amalgams be subjected to the important and convenient step of phase exchange because such reactions are very slow owing to the nature of the amalgams. For instance, if zinc amalgam is contacted by a copper sulfate solution, a phase exchange reaction will immediately ensue, which in the ideal case results in the formation of copper amalgam and zinc sulfate solution. This reaction proceeds with increasing intensity, which may be illustrated by comparing it to an electric current flow of equivalent effect. For instance, the intensity of the Zn-Cu phase exchange corresponds to a current flow of several thousand amperes per square meter through the phase boundary between amalgam and aqueous solution. On the other hand, if nickel or iron amalgam is contacted by a copper sulfate solution, the phase exchange is so slow that it can only be compared to a current flow of several amperes per square meter, which is inadequate for industrial operation.

For this reason a processing of the amalgams of the iron metals was previously possible only by a heat treatment (distillation). This method is not only difficult as far as the necessary apparatus is concerned but is also uneconomical because in automatic operation, which is necessary in most cases, smaller units require structural and financial expenditures which are not in proportion with the output of such units.

Iron amalgam is formed as an intermediate product in various electrochemical processes. The most important of these processes is certainly the electrolytic regeneration of spent pickling acid solutions for iron by means of mercury cathodes to form iron amalgam and free acid. Besides, salt solutions of noble metals, such as aluminium sulfate solutions, can easfly be freed from their undesired contents of nobler metals (Fe, Cu, Ni, Zn) by an electrolysis on mercury cathodes. For this reason it was obvious and, in view of the suggested difficulties during the thermal decomposition, actually necessary to develop an aqueous process for decomposing iron amalgam.

A step towards solving the problem is provided by the process described in the U.S. patent specification No.

1,970,973- (Palmaer), in which iron amalgam is to be purified by a reaction with an acidaqueous solution of an oxidizing agent, such as ferric sulfate.

According to the equation this process results in the decomposition of iron amalgam at an industrially useful rate. However, the subsequent additional oxidation of the iron to which the iron removed from the amalgam is subjected according to said patent would result in the formation of an equivalent amount of ferrous sulfate, which is not usable and, on the other hand, combines acid so that the process would not be suitable for the regeneration of acid. In fact, the process of Palmaer is only contemplated for the removal of heavy metals from salt solutions of base metals for cleaning purposes. On the other hand, if it is desired to effect an electrolytic precipitation of iron from highly acid solutions and to increase the acidity of the latter, it is neces sary to remove the iron in elementary form from the process, as is effected by the method of the invention.

The process according to the invention for processing iron-containing amalgams resides in that iron-containing amalgam, formed in electrolytic cells on mercury cathodes (primary cells), is contacted or leached with a solution which contains ferric sulfate and ferrous sulfate, the amount of ferrous sulfate being in excess of the amount of ferric sulfate. The resulting solution, which contains predominantly ferrous sulfate, is fed into the cathode spaces of electrolytic cells (secondary cells), which are equipped with solid metal cathodes and deteriorationresisting anodes separated by filter diaphragms from the cathodes. The electrolyte in the secondary cells flows from the cathodes to the anodes and is withdrawn or flows from the anode space as a solution containing ferric sulfate and ferrous sulfate, the latter being in excess of the former. This electrolyte is then recycled to the iron-containing amalgam as leaching agent.

It has proved desirable to effect the reaction of the solution which contains ferric and ferrous salt with the ironcontaining amalgam at an elevated temperature, preferably between 40 and C., with a vigorous agitation of the phase boundary between amalgam and aqueous solution and at a pH between 0 and 4.

The mercury and aqueous phases are preferably reacted under conditions of parallel or antiparallel flow in towers which are empty or filled with packing bodies and/ or in trough-shaped devices which are horizontal or slope in the direction of flow of the mercury and are empty or filled with baffle bodies and/or in stirredvessels (reaction vessels).

To prevent an oxidative deterioration of the mercury by the solution containing ferric and ferrous salt the iron content of the amalgam should not be allowed to drop excessively. For this reason the amalgam must have an iron content of at least 0.05% after the reaction with the aqueous phase has been completed.

Any small residual content of ferric ions in the reacted electrolyte will be consumed by additionally reducing the ferrous salt solution with metallic iron after the solution has left the reaction vessels. This is preferably effected by passing the solution through towers filled with scrap iron. The scrap towers are connected between the reaction vessels and the secondary electrolytic cells.

In order to remove any small quantities of mercury which have also passed into the solution which contains ferrous sulfate, this solution is contacted with copper after the treatment with the scrap iron. This process step is effected by passing the electrolyte through a tower filled with copper chips after the electrolyte has left the tower filled with scrap iron. Mercury is deposited on the copper or copper amalgam surface of the copper chips whereas an equivalent amount of copper enters the solution. In order to remove impurities, e.g., carbon introduced by the scrap iron, the electrolyte which has left the tower filled with scrap iron or scrap copper is filtered before it enters the cathode spaces of the secondary cells.

The velocity of flow of the electrolyte from the oathode spaces to the anode spaces of the secondary cells through the filter diaphragms is preferably higher than the velocity of migration of the fastest cation in the electrical field in the electrolyte at the operating temperature (up to 100 C.). This will prevent an entrance of hydrogen or ferric ions from the anode spaces into the cathode spaces, where they could cause disturbances by a reduction of the current efficiency or a discharg This flow of electrolyte is maintained by maintaining the liquid level in the cathode spaces higher than in the anode spaces. This can be effected by known methods, e.g., by a regulation of the liquid level in the cathode spaces or in the anode spaces or in both at the same time.

The primary and secondary cells are preferably electrically connected in series. To compensate the differences between the electrolytic efliciences of both kinds of cells either a part of the current is conducted, through a resistor which shunts the cell which has a higher cathodic current efficiency or, if the secondary cell has higher cathodic and anodic current efliciences, the solution which is formed in the secondary cell and contains ferric and ferrous sulfate is not entirely reduced with iron-containing amalgam but a part of this solution is branched off before and directly fed to the scrap iron tower. If the anodic current efficiency in the secondary cell (with respect to the formation of ferric ions) exceeds the cathodic current efficiency (with respect to discharged iron), the correction is effected by an addition of acid and possibly by a reduction of the iron salt concentration of the electrolyte. In most cases it is necessary to adopt these methods of compensation simultaneously and/ or in combination.

The process can be carried out on an industrial scale as follows:

A mercury cathode cell (primary cell), which processes a spent iron containing sulfate pickling solution to form free acid and iron amalgam (regeneration of spent pickle), is electrically connected in series with an electrolytic cell (secondary cell) having sheet iron cathodes, filter diaphragms of plastic and lead anodes. The mercury cathode cell is divided by a diaphragm into a cathode compartment and an anode compartment. Spent iron containing sulfate pickling solution is fed into the cathode compartment while diluted sulfuric acid is introduced into the anode compartment. Sulfuric acid is formed at the anode due to discharge of sulfate ions and thus this formed acid concentrates the diluted sulfuric acid originally introduced into the anode compartment. The mercury of the primary cell is recycled through the cell, a mercury pump and a reaction tower, filled with glass beads, at a rate which causes it to charged with 1% iron during each pass through the cell. The iron content of the amalgam during operation is preferably (LS-0.8% Fe.

At the same time the anolyte flows from the secondary cell in an antiparallel flow with respect to the mercury. The anolyte contains per liter, e.g., 60 grams iron as sulfate, 20 grams ammonium sulfate and suitably 0.1 gram of the wetting agent Erkantol BX. The reacted amount of ferric ions is sufficient to remove 0.1% iron from the amalgam flowing through. The anolyte which has now been partially reduced is subsequently passed through the scrap and copper towers, filtered and charged into the cathode space of the secondary cell. Having become a catholyte, about 10 grams of iron per liter are removed from the solution by the cathodic discharge of iron before the solution flows through the diaphragm to the anode. Twice the amount of iron, e.g., 20 grams Fe/liter is additionally oxidized at the same time to form ferric sulfate. The residual iron remains in the electrolyte as ferrous sulfate. This means in fact that the electrolyte contains a substantial excess of ferrous sulfate in relation to ferric sulfate, the ratio ferrous ions to ferric ions being in this embodiment about 2:1. This anolyte is conveyed by a pump back to the reaction tower.

If the cathodic current efficiency of the primary cell and the cathodic and anodic current efficiencies of the seconary cell is 100% with respect to the formation of amalgam, discharge of iron and formation of ferric ions, respectively, the discharge of iron as amalgam in the primary cell will correspond to the discharge of an equal amount of electrolytic iron in the secondary cell, in which the correspondingly required amount of ferric sulfate is also formed. This ideal case, however, cannot be achieved in practice. All three electrode processes described are performed with different current efficiencies. In the primary cell a current efficiency of can be practically expected whereas the anode and cathode of the secondary cell operate with a current efficiency of more than To compensate this difference the secondary cell may be shunted by a rheostat to branch off a corresponding part of the overall current before this cell; this would mean dissipation of energy or a corresponding part of the anolyte formed in the secondary cell is not passed through the reaction tower but is directly conveyed through the scrap tower to the secondary cell. Whereas this involves a higher consumption of scrap it results in a production of additional valuable electrolytic iron by the energy which would otherwise be dissipated. The compensation is controlled by checking the iron content of the amalgam from time to time and controlling the amount of anolyte branched off to prevent the iron content of the amalgam from dropping below 0.5% and from rising about 0.8%.

If the ratio of the cathodic current eflicencies of the two cells is known and the branched-off electric current or electrolyte has been regulated approximately in accordance therewith, a daily analytic check and adjustment will be entirely su.ncient. Owing to the considerable amount of iron in the circulating volume of amalgam the changes in the iron contents are very slow.

The different current efiiciencies at the cathode and anode of the secondary cell must also be taken into consideration. When the current efficiency of the cathode is higher than that of the anode, the acidity of the anolyte will increase and will contain less ferric ions. In this case the regulation is effected by a reduction of the amount of anolyte branched off to the scrap tower. On the other hand, the liberated acid consumes somewhat more scrap to saturate itself. If the saturation in the scrap toweris not sufiiciently complete, the higher acid content in the catholyte will cause a certain reduction of the cathodic current efficiency, whereby the current efficiencies are automatically adjusted to each other.

However, if the cathodic current efficiency of the secondary cell is lower than the anodic current efficiency therein, the content of free acid in the electrolyte will be reduced whereas the content of iron ions will increase at the same time. Any corrections required in this case will be effected by an addition of sulfuric acid and possibly by transferring a part of the electrolyte into the cycle of the electrolyte of the primary cell and replacing this transferred electrolyte by water.

in most cases a regulation to compensate the different current efiiciencies in the secondary cell are not necessary because the current efficiency on the cathode is highly dependent on the acid content of the electrolyte whereby the system is highly self-regulating, the cathodic current ef-. ficiency adjusting itself to the anodic one.

As will have become evident from the above, the process according to the invention for the continuous electrolytic regeneration of spent iron-containing sulfate picklingsolutions comprises essentially four steps which may be briefly summarized as follows:

(1) The pickling solution is first subjected to electrolysis in an electrolytic cell having an anode and a mercury cathode. In this manner free sulfuric acid is formed at the anode and iron amalgam is formed of the cathode.

(2) The iron amalgam is then leached with an aqueous solution containing both ferric sulfate and ferrous sulfate. The amount of ferrous sulfate is in excess of the amount of ferric sulfate. According to a preferred embodiment the ratio of ferrous ions to ferric ions is about 2:1. The aqueous solution should also preferably contain a wetting agent. The leaching of the iron amalgam with the aqueous solution thus described must be effected with attendant vigorous agitation of the phase boundary between the iron amalgam and the aqueous solution. In this manner a solution is formed which predominantly contains ferrous sulfate. The aqueous leaching solution containing the ferric sulfate and ferrous sulfate is employed in such quantities so that the concentration of the iron in the iron amalgam after the leaching is at least 0.05%.

(3) The solution thus obtained is then fed into a cathodic zone of a secondary electrolytic cell having an iron cathode and a deterioration resistant anode with a porous diaphragm therebetween, thereby defining a cathodic zone and an anodic zone. A flow of the ferrous sulfate containing solution, to wit, the electrolyte, is maintained from the cathodic zone through the porous diaphragm to the anodic zone. Concurrently therewith a negative potential is maintained on the cathode of the secondary electrolytic cell and a positive potential on the anode of the secondary electrolytic cell. In this manner a portion of the ferrous ions in the electrolyte are converted to forric ions. An aqueous solution is thus obtained which contains both ferric sulfate and ferrous sulfate, the latter being in excess of the former.

(4) This solution is then removed from the anodic zone of the secondary electrolytic cell and is recycled for leaching of iron amalgam in step (2).

What we claim is:

1. A process for the continuous electrolytic regeneration of spent iron containing sulfate pickling solution which comprises the combination of the following four process steps, to wit:

(1) subjecting the pickling solution to electrolysis in an electrolytic cell having an anode and a mercury cathode, whereby free sulfuric acid is formed at the anode and iron amalgam is formed of the cathode,

(2) leaching the iron amalgam with an aqueous solution of ferric sulfate and ferrous sulfate, with the amount of ferrous sulfate being in excess of the amount of ferric sulfate, said leaching being effected with attendant vigorous agitation of the phase boundary between the iron amalgam and said aqueous solution to form a ferrous sulfate electrolytic solution, said aqueous solution being employed in quantity whereby the concentration of the iron in the iron amalgam after the leaching is at least 0.05 percent,

(3) thereafter feeding the thereby produced ferrous sulfate containing electrolyte into a cathodic zone of a secondary electrolytic cell having an iron cathode and a deterioration resistant anode with a porous diaphragm therebetween, thereby defining a cathodic zone and an anodic zone, maintaining a flow of said electrolyte from said cathodic zone through said diaphragm to said anodic zone, concurrently with said flow maintaining a negative potential on said cathode of said secondary electrolytic cell and a positive potential on said anode of said secondary electrolytic cell, thereby converting a portion of the ferrous ions in the electrolyte to ferric ions to obtain an aqueous solution which contains ferric sulfate and ferrous sulfate, with the latter in excess of the former, and

(4) thereafter removing said aqueous solution from the anodic zone of the said secondary electrolytic cell and recycling said aqueous solution to step (2) for leaching of iron amalgam.

2. A process as claimed in claim 1, wherein said iron amalgam is leached with said aqueous solution at a tem perature of between about 40-100 C. and a pH value of between about 0 to 4.

3. A process as claimed in claim 1, wherein the anode of said secondary electrolytic cell is of lead.

4. A process as claimed in claim 1, wherein the ferrous sulfate solution obtained in step (2) is contacted with iron before being fed into said cathodic zone of step (3).

5. A process as claimed in claim 4, wherein the ferrous sulfate solution is contacted with copper after having been contacted with iron and before being fed into said cathodic zone of step (3).

6. A process as claimed in claim 1, wherein the ratio of ferric ions to ferrous ions in said aqueous solution is about 1:2.

7. A process as claimed in claim 1, wherein said aqueous solution contains a minor amount of a wetting agent.

8. A process for the continuous electrolytic regeneration of spent iron containing sulfate pickling solution which comprises the combination of the following four process steps, to wit:

(1) subjecting the pickling solution to electrolysis in an electrolytic cell having an anode and a mercury cathode, whereby free sulfuric acid is formed at the anode and iron amalgam is formed of the cathode,

(2) leaching the iron amalgam with an aqueous solution of ferric sulfate and ferrous sulfate, wherein the ratio of ferrous ions to ferric ions is about 2: 1, said aqueous solution containing a minor amount of a wetting agent, said leaching being effected with at tendant vigorous agitation of the phase boundary between the iron amalgam and said aqueous solution to form a ferrous sulfate electrolytic solution, said aqueous solution being employed in quantity whereby the concentration of the iron in the iron amalgam after the leaching is at least 0.05%

(3) thereafter feeding the thereby produced ferrous sulfate containing electrolyte into a cathodic zone of a secondary electrolytic cell having an iron cathode and a deterioration resistant anode with a porous diaphragm therebetween, thereby defining a cathodic zone and an anodic zone, maintaining a fiow of said electrolyte from said cathodic zone through said diaphragm to said anodic zone, concurrently with said flow maintaining a negative potential on said cathode of said secondary electrolytic cell and a positive potential on said anode of said secondary electrolytic cell, thereby converting a portion of the-ferrous ions in the electrolyte to ferric ions to obtain an aqueous solution which contains ferric sulfate and ferrous sulfate with the ratio of ferrous ions to ferric ions being about 2:1, and

(4) thereafter removing the said aqueous solution from the anodic zone of the said secondary electrolytic cell and recycling said aqueous solution for leaching of iron amalgam.

References Cited by the Examiner UNITED STATES PATENTS 1,970,973 8/1934 Palrnear 204124 2,273,798 2/ 1942 Heise et al 20482 2,810,686 10/1957 Bodamer et al 2041 30 JOHN H. MACK, Primary Examiner.

H. M. FLOURNOY, Assistant Examiner. 

1. A PROCESS FOR THE CONTINUOUS ELECTROLYTIC REGENERATION OF SPENT IRON CONTAINNG SULFATE PICKLING SOLUTION WHICH COMPRISES THE COMBINATION OF THE FOLLOWING FOUR PROCESS STEPS, TO WIT: (1) SUBJECTING THE PICKLING SOLUTION TO ELECTROLYSIS IN AN ELECTROLYTIC CELL HAVING AN ANODE AND A MERCURY CATHODE, WHEREBY FREE SULFURIC ACID IS FORMED AT THE ANODE AND IRON AMALGAM IS FORMED OF THE CATHODE, (2) LEACHING THE IRON AMALGAM WITH AN AQUEOUS SOLUTION OF FERRIC SULFATE AND FEROUS SULFATE, WITH THE AMOUNT OF FERROUS SULFATE BEING IN EXCESS OF THE AMOUNT OFFERRIC SULFATE, SAID LEACHING BEING EFFECTED WITH ATTENDANT VIGOROUS AGITATION OF THE PHASE BOUNDARY BETWEEN THE IRON AMALGAM AND SAID AQUEOUS SOLUTION TO FORM A FERROUS SULFATE ELECTROLYTIC SOLUTION, SAID AQUEOUS SOLUTION BEING EMPLOYED IN QUANTITY WHEREBY THE CONCENTRATION OF THE IRON IN THE IRON AMALGAM AFTER THE LEACHING IS AT LEAST 0.05 PERCENT. (3) THEREAFTER FEEDING THE THEREBY PRODUCED FERROUS SULFATE CONTAINING ELECTROLYTE INTO A CATHODIC ZONE OF A SECONDARY ELECTROLYTIC CELL HAVING AN IRON CATHODE AND A DETERIORATION RESISTANT ANODE WITH A POROUS DIAPHRAGM THEREBETWEEN, THEREBY DEFINING A CATHODIC ZONE AND AN ANODIC ZONE, MAINTAINING A FLOW OF SAID ELECTROLYTE FROM SAID CATHODIC ZONE THROUGH SAID DIAPHRAGM TO SAID ANODIC ZONE, CONCURRENTLY WITH SAID FLOW MAINTAINING A NEGATIVE POTENTIAL ON SAID CATHODE OF SAID SECONDARY ELECTROLYTIC CELL AND A POSITIVE POTENTIAL ON SAID ANODE OF SAID SECONDARY ELECTROLYTIC CELL, THEREBY CONVERTING A PORTION OF THE FERROUS IONS IN THE ELECTROLYTE TO FERRIC IONS TO OBTAIN AN AQUEOUS SOLUTION WHICH CONTAINS FERRIC SULFATE AND FERROUS SULFATE, WITH THE LATTER IN EXCESS OF THE FORMER, AND (4) THEREAFTER REMOVING SAID AQUEOUS SOLUTION FROM THE ANODIC ZONE OF THE SAID SECONDARY ELECTROLYTIC CELL AND REBYBLING SAID AQUEOUS SOLUTION TO STEP (2) FOR LEACHING OF IRON AMALGAM. 